Escorted by his bodyguards, Iranian President Mahmoud Ahmadinejad, center, embraces a young girl, during a rally commemorating anniversary of the 1979 Islamic Revolution, in Tehran, Iran, Thursday, Feb. 11, 2010. Ahmadinejad claimed Thursday that Iran has produced its first batch of uranium enriched to a higher level just two days after it said it began the process, showcasing new advances in its controversial nuclear program. (AP Photo/Vahid Salemi)
(Vahid Salemi - AP)

Escorted by his bodyguards, Iranian President Mahmoud Ahmadinejad, center, is greeted by demonstrators as he attends a rally commemorating anniversary of the 1979 Islamic Revolution, in Tehran, Iran, Thursday, Feb. 11, 2010. Ahmadinejad claimed that Iran has produced its first batch of uranium enriched to a higher level just two days after it said it began the process, showcasing new advances in its controversial nuclear program. (AP Photo/Vahid Salemi)
(Vahid Salemi - AP)

Iranian President Mahmoud Ahmadinejad, talks to supporters during a rally commemorating the anniversary of the 1979 Islamic Revolution, at the Azadi (freedom) Square in Tehran, Iran, Thursday, Feb. 11, 2010. Ahmadinejad claimed Thursday that Iran has produced its first batch of uranium enriched to a higher level, just two days after it said it began the process, showcasing new advances in its controversial nuclear program. Many thousands of pro-government demonstrators gathered in the central square of Tehran to mark the 31st anniversary of Iran's 1979 Islamic Revolution that toppled the US-backed late Shah Mohammad Reza Pahlavi. (AP Photo/Vahid Salemi)
(Vahid Salemi - AP)

Female demonstrators wave Iranian flags, as a picture of supreme leader Ayatollah Ali Khamenei, is held at center, during a rally commemorating the anniversary of the 1979 Islamic Revolution, at the Azadi (freedom) Square in Tehran, Iran, Thursday, Feb. 11, 2010. Many thousands of pro-government demonstrators gathered in the central square of Tehran to mark the 31st anniversary of Iran's 1979 Islamic Revolution that toppled the US-backed late Shah Mohammad Reza Pahlavi. (AP Photo/Vahid Salemi)
(Vahid Salemi - AP)

TEHRAN -- Hundreds of thousands of government supporters massed Thursday in central Tehran to mark the 31st anniversary of Iran's Islamic revolution, while a heavy deployment of security forces largely prevented protesters from using the occasion to stage opposition rallies.

The protesters were met by unusually large security forces, which closed off streets and cut access to groups of opposition supporters to the central avenue and square where the anniversary was celebrated. Witnesses reported clashes in the Sadeghiyeh neighborhood, where opposition leader Mehdi Karroubi tried to join the rallies.

He was forced to leave when security forces affiliated with the Revolutionary Guard Corps used tear gas to disperse crowds. The windows of his car were smashed by plainclothes members of a paramilitary unit, witnesses said.

Revolutionary Guard special forces, clad in olive uniforms and black ski masks, carried assault rifles as they directed groups of soldiers, and there were unconfirmed reports of shootings. A spokesman for Karroubi said that a tear gas grenade exploded in front of the former speaker of parliament and that his eyesight was damaged.

International media representatives were prevented from freely covering the rally and were placed in a special area surrounded by government supporters holding up posters showing Iran's supreme leader, Ayatollah Ali Khamenei.

In Washington, a State Department spokesman said the Ahmadinejad government has attempted to choke off the flow of information within Iran, adding, "It is clear . . . the Iranian government fears its own people."

The spokesman, P.J. Crowley, said he had seen reports that Iran's telephone network, text messaging, satellite television and the Internet had all been jammed. "It would appear that Iran has attempted a near total information blockade," he said. White House spokesman Robert Gibbs said Google and other Internet service providers had been "unplugged" in Iran.

Opposition activists and Web sites nevertheless reported that plainclothes agents beat an opposition leader's wife with batons during Thursday's demonstrations and that security forces briefly detained the granddaughter of Ayatollah Ruhollah Khomeini, the father of the Islamic revolution, along with her husband. There was no immediate official comment on the alleged beating of Zahra Rahnavard, 65, the wife of former opposition presidential candidate Mir Hossein Mousavi, or on the detention of Zahra Eshraghi and her husband, Mohammad Reza Khatami, both prominent reformists. The couple were held for less than an hour before being released, news agencies reported.

The opposition Web site Kaleme said protesters who tried to march toward Azadi Square while waving green flag and chanting, "Death to the dictator," were dispersed by volleys of tear gas fired by security forces. A number of protesters were arrested, Kaleme said.

"What problems do the anti-government protesters have in the Islamic Republic?" asked Zahra Farahani, a government supporter who came by bus from the Shiite holy city of Qom. "There are no problems here. They are influenced by foreigners and must return to the path of our dear leader," she said as girls around her waved Iran's green, white and red flag.

Dozens of paratroopers descended from the blue winter sky, long Iranian flags attached to their parachutes. On one of the streets leading to the central Azadi Square, a Safir rocket, used to launch Iranian satellites, was put on display.

Even at the square, the heart of the rally, security measures were tight. Government supporters were divided over several areas cordoned off with walls made of scaffolding.